If you’re using a generator to recharge a lithium battery bank in your caravan, the charger sitting between the two matters just as much as the generator itself.
A common setup is a 2000W inverter generator connected to a 12V 100A AC battery charger. The charger takes the generator’s 240V AC output and converts it into DC power for the caravan’s lithium battery. On paper, 12V × 100A looks like a 1,200W load. In practice, the charger can demand considerably more from the generator once charging voltage and conversion losses are taken into account.
That can be enough to overload a 2000W generator. The engine may bog down, the overload light can come on, and the generator may cut the output.
Nothing is necessarily wrong with either piece of equipment. The problem is that the charger can ask for more power than the generator can supply.
Here’s how the numbers work, what to check before pairing a charger with a generator, and what you can do if your setup is undersized.
» MORE: Compare rated outputs across the Outbax inverter generator range
Why Does a 100A Charger Overload a 2000W Generator?
A 100A charger overloads a 2000W inverter generator because its peak demand on the 240V side is far higher than the label suggests. Three separate multipliers stack up, and none of them appears on the box.
1. A 12V lithium battery does not charge at 12V. Lithium iron phosphate, or LiFePO4, accepts bulk charge at roughly 14.2V to 14.6V. To push 100A into the bank, the charger has to deliver that current at the higher voltage:
100A × 14.6V = 1,460W of DC output
2. The charger is not perfectly efficient. A quality 12V 100A multistage mains charger converts 240V AC to regulated DC at roughly 85 to 90 per cent efficiency. The rest becomes heat, which is why these units have fans. At 85 per cent, the charger has to draw more than it delivers:
1,460W ÷ 0.85 = about 1,718W of AC input
3. The generator is limited by volt-amps, not watts. This is the step almost everyone misses. If the charger has no power factor correction, its power factor can be as low as 0.70. An inverter generator is constrained by total apparent power, measured in volt-amps:
1,718W ÷ 0.70 = about 2,454VA
One further point: the Gentrax specification sheets for the GTX6600 and the GT3500 family both list Power Factor 1.0. The rated output is quoted at unity, so no hidden volt-amp reserve sits above the watt figure. When a load presents 2,454VA to a machine rated for 1,700W, the electronic breaker does exactly what it was designed to do.
Rated Output Is the Number That Matters, Not Maximum Output
Maximum output is a brief surge figure. Rated output is what the machine holds continuously, typically 10 to 20 per cent lower. The Gentrax manuals are blunt: exceeding the rated load will damage the unit, shorten its life and invalidate the guarantee.
Here is how five Gentrax inverter generators stocked by Outbax actually compare on the figures that decide whether your charger runs.
| Model | Max output | Rated output | 12V DC socket | Fuel tank | Run time | Starting |
|---|---|---|---|---|---|---|
| Gentrax GT2000 | 2.0kW | 1.7kW | 12V / 5.0A | 4.1 L | 6.1 hrs at 50% load | Recoil |
| Gentrax GT3500 | 3.5kW | 3.0kW | 12V / 8.3A or 5.0A | 5.7 L | 6.2 hrs at 50% load | Recoil |
| Gentrax GTX3500 | 3.5kW | 3.2kW | 12V / 5.0A | 7.5 L | 6.3 hrs at 50% load | Recoil, electric, remote |
| Gentrax GTX4200 Pro | 4.2kW | 3.5kW | 12V / 8A | 8.8 L | 6.8 hrs at 50% load | Recoil, electric, remote |
| Gentrax GTX6600 | 6.6kW | 6.0kW | 12V / 5.0A plus 5V / 3.1A | 18.5 L | 6.6 hrs at 50% load | Electric, remote |
Five Ways to Stop the Overload
1. Turn the Charger’s Output Current Down
Most modern 100A smart chargers let you cap maximum current through an app or a physical switch. Drop the limit to 60A or 70A and demand falls into a range a small generator can hold. This costs nothing, works immediately and should be your first move.
2. Connect the Load Before You Switch Eco Mode Back On
The Gentrax manuals are specific here. Always allow the generator to reach full operating speed before connecting any electrical load, and turn the economy control switch to OFF when running devices that need a large starting current. Eco mode drops engine speed at idle, so a heavy load landing on a lazy engine is far more likely to trip the breaker than the same load landing on one already at full revs. Start the generator, let it settle, switch eco off, connect the charger, then try eco again once the charge is stable.
3. Switch Every Other 240V Load Off
A 100A charger leaves no headroom. The 240V compressor fridge, the air conditioner, and the hot water element all have to be off or running on gas. On an Australian summer afternoon, the air conditioner alone can account for more than a small generator’s entire rated output.
4. Reset the Generator Properly After a Trip
On the GTX3500, GTX4200 Pro, and GTX6600, holding the output reset button for two seconds restores AC output once the fault is cleared. The GT2000 has no reset button, so you stop and restart the engine. Reduce the load first either way. The manuals also note that the overload light showing for a few seconds at startup under a heavy load is normal, as long as it goes out again.
5. Buy the Right Hardware for the Job
If the settings fixes are not enough, replace the charger with one that has active power factor correction, which holds power factor near 0.98 and keeps volt-amp demand close to true wattage. The alternative is a bigger generator. The charger is usually cheaper.
What Size Generator Do You Need for a 100A Charger?
The table below shows the calculations for each charger setting, using a poor power factor of 0.70 and a corrected power factor of 0.98.
| Charger current | DC output at 14.6V | AC input at 85% | Apparent power at PF 0.70 | Apparent power at PF 0.98 | Suggested rated output |
|---|---|---|---|---|---|
| 40A | 584W | 687W | 981VA | 701VA | 1.7kW is workable |
| 60A | 876W | 1,031W | 1,472VA | 1,052VA | 1.7kW is tight, 3.0kW is comfortable |
| 80A | 1,168W | 1,374W | 1,963VA | 1,402VA | 3.0kW or above |
| 100A | 1,460W | 1,718W | 2,454VA | 1,753VA | 3.0kW or above |
For Australian touring, the conclusion is simple. If you want a 100A charger at full output from petrol, 3.5kVA class is the entry point: a Gentrax GTX3500 at 3.2kW rated, or a GT3500 at 3.0kW rated, absorbs it even with a mediocre power factor. If the air conditioner also has to run, the GTX6600 at 6.0kW rated is the honest answer.
Noise matters too. Many caravan parks and national parks enforce generator curfews, and a larger machine at half load is usually quieter than a small one running flat out.
Charging LiFePO4 from a Generator Without Damaging Anything
The overload is worst at the start of a charge. In the bulk phase, the battery accepts everything the charger provides, so demand stays at maximum. Once the bank reaches absorption voltage and enters the saturation phase, current tapers and the generator settles. This is why some owners see the fault only when the bank is low, and conclude the generator is faulty when it is not.
The Battery Management System sets the ceiling on what the pack accepts, and will limit or cut current if cell temperature or voltage moves outside its window. On a cold morning, a pack may refuse charge until it warms.
For run time, budget realistically. Taking a 200Ah lithium bank similar to the VoltX 12V 200Ah Pro Lithium Battery from 20 per cent back to full means replacing roughly 160Ah. At 60A, that is close to three hours in bulk, plus a further hour or more as the charge tapers. Generators are a top-up tool for a run of overcast days, not a substitute for solar.
Do Not Charge Lithium Through the Generator’s 12V DC Socket
Every Gentrax manual describes the DC outlet as suitable for 12V battery charging only, then walks through loosening the battery vents, topping up with distilled water and checking specific gravity with a hydrometer until it reads 1.26 to 1.28. That is a flooded lead-acid procedure. A sealed LiFePO4 pack has no fluid to test, and at 12V and 5.0A the socket delivers around 60W, which is a trickle. Charge lithium through a proper 240V charger plugged into the AC receptacle, or through a DC-DC charger from the vehicle.
The Bottom Line
The stall is a unit’s problem, not a fault. A 100A charger can ask for around 2,450VA, while a 2000W inverter generator holds 1,700W. Before you replace anything, lower the charger’s current limit, switch off the other 240V loads, and connect the load with eco mode off. If you still want full 100A charging from petrol, move up to the 3.5kVA class and size for the loads you actually run. The rated output figures for every model are published on the Outbax product pages, and they are the numbers worth comparing.
